Aqueous solution of CoCl2 contains [Co(H2O)6]2+ which is pinkish in colour so option (d) is incorrect. Reduction potential of Co3+⟶Co2+ is high so option(b) is incorrect. Co2+ does not oxidises easily to Co3+. It is general case that symmetrical substituted octahedral complexes are less deeper in colour than tetrahedral complexes. So [CoCl4]2− is deep blue in colour.